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Rotating Satellite Interferometer for Direction Finding

Publishing Venue

IBM

Related People

Raabe, HP: AUTHOR

Abstract

This is an active spinning interferometer radiating a signal into space from which a receiver can derive the angular coordinates of its position with respect to the position and attitude of the interferometer. The interferometer is mounted on a spinning synchronous satellite and the signal received on the ground can be used for navigation. Being an active source, an unlimited number of users can participate. The spinning axis is normal to the orbital plane and the interferometer may thus be a rigid part of a spin-stabilized satellite.

Country

United States

Language

English (United States)

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Rotating Satellite Interferometer for Direction Finding

This is an active spinning interferometer radiating a signal into space from
which a receiver can derive the angular coordinates of its position with respect to
the position and attitude of the interferometer. The interferometer is mounted on
a spinning synchronous satellite and the signal received on the ground can be
used for navigation. Being an active source, an unlimited number of users can
participate. The spinning axis is normal to the orbital plane and the
interferometer may thus be a rigid part of a spin-stabilized satellite.

When the base line of the interferometer is tilted at an arbitrary angle,
preferably 45 degrees with respect to the spin axis, the interferometer radiation
pattern will generate signals on the ground whose modulation is an unambiguous
function of the position of the receiver. The system is especially useful when the
satellite is in an equatorial synchronous orbit.

In Fig. A, center body 1 has two arms, 2 and 3, which support radiators 4
and 5 and balancing masses 6 and 7. The arms also incorporate RF
transmission lines connecting the radiation to a power source located in the
center body. The mass distribution is designed so that the satellite's highest
moment of inertia is aligned with the body of axis S-S. The satellite is injected
into the synchronous orbit with a spin about axis S-S so that it is normal to the
plane of the orbit.

The patterns of radiators 4 and 5 should be identical in field strength and
polarization and do not have to cover more than the solid angle subtended by the
earth, which is about 17 degrees wide. Because despinning of the radiators is not
very practical, biconical horn antennas, generating a doughnut-shaped pattern,
are used. The polarization of the radiation is parallel to the spin axis.

When the radiators operate in phase, the interferometer pattern consists of
2n + 1 lobes with n = b/Lambda, where b is the distance between the radiators.
The peaks are aligned with as many concentric cones as lobes around the base
line axis W-W. Thus, the lobes are separated by 2n nodes where the
interferometer field vanishes. If radiators 4 and 5 are fed with out-of-phase
signals, 2n lobes are generated. They will also be aligned with as many
concentric cones. Thy two sets of lobes will interleave in such a way that the
peak of one set will be found in the directions of the nodes of the other set.

As the interferometer rotates with the spinning satellite, the cone patterns
cause a F-M periodic amplified modulation of the received RF signal. By
geometry each cycle of satellite rotation represents a unique signature of the
location P of a receiver on the earth's surface.

The angle Alpha defines the cone pattern with respect to an orthogonal
coordinate system established by the spin axis S-S, the line X directed toward
the center of the earth and the line VEL which is tangential to the circular
synchronous orbit of the satellite.